Inbred SENCARA/Pt mice are very susceptible to the induction of both benign and malignant skin tumors by standard initiation-promotion protocols. Crosses between SENCARA/Pt and resistant BALB/cAnPt mice revealed that resistance to papilloma induction is incompletely dominant, and that multiple genes determine susceptibility. A genome scan led to the identification of a major locus controlling susceptibility on chromosome 5 in a 21 cM interval between D5Mit259 and D5Mit369, and modifier loci on chromosomes 9, 11 and 12. A large series of backcross mice has been bred to determine whether the genes responsible for development of benign and malignant skin tumors are the same or different. In this study, F1 progeny produced by using either SENCARA/Pt or BALB/cAnPt females were uniformly resistant to skin tumor induction. However, the susceptibility phenotype of backcross mice that resulted from the cross of F1 females to SENCARA/Pt males depended on the strain of the females used to produce the F1 progeny. When SENCARA/Pt females were used (compared to BALB/cAnPt females) to produce F1 mice, about 45 % of the backcross mice were susceptible (compared to 15 % for BALB/cAnPt). The most straightforward explanation of this result is that a susceptibility gene resides on the X chromosome. We have demonstrated a marked synergism for tumor promotion between the phorbol ester 12-O-tetradecanoyphorbol-13-acetate (TPA) and agents that elevate intracellular calcium, such as thapsigargin or the calcium ionophore ionomycin, suggesting a role for calcium in tumor promotion by phorbol esters. Synergism was not found at effective promoting doses of TPA, but was seen at suboptimal or non-promoting doses of TPA. The mechanism by which elevation of calcium increases promotion by TPA may involve a conventional isoform of protein kinase C (such as protein kinase C alpha) or another calcium-activated pathway. Activating mutations in the K-ras oncogene, rarely seen in mouse skin tumors, are frequent in rapidly-arising papillomas and carcinomas on the skin of mice initiated by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and promoted by mezerein. Only a few K-ras mutations were seen in skin tumors from MNNG-initiated mice promoted with TPA. Mouse skin tumors initiated by 7,12-dimethylbenz[a]anthracene and promoted by either TPA or mezerein display mutations in H-ras but not K-ras. The mechanism of the selective action of mezerein in the promotion of MNNG-initiated skin tumors may be related to the K-ras and mezerein-activated protein kinase C pathways in skin. We have analyzed skin tumors from this experiment for hypermethylation of the promoter for the gene encoding the DNA repair enzyme MGMT, and found hypermethylation in 41 % of papillomas and 51 % of carcinomas. However, we found that methylation was not preferentially associated with tumors with either H- or K-ras mutations.